selftests: netconsole: Add tests for 'release' feature in sysdata

Expands the self-tests to include the 'release' feature in
sysdata.

Verifies that enabling the 'release' feature appends the
correct data and ensures that disabling it functions as expected.

When enabled, the message should have an item similar to in the
userdata: `release=$(uname -r)`

Signed-off-by: Breno Leitao <leitao@debian.org>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://patch.msgid.link/20250314-netcons_release-v1-5-07979c4b86af@debian.org


Signed-off-by: Paolo Abeni <pabeni@redhat.com>